ringaccount: improve loading logic

* setup trust store when creating device
* fix createAccount, migrateAccount to use loaded id
  since identity_ is not set anymore
* useIdentity: loadIdentity will now fail when migration is needed,
  adapt logic. Recursively call loadIdentity when migration has ended.

Change-Id: Ie4f993fdd13fac894679b96a20fc5eacff5817c3
Tuleap: #1457